Telephone system



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wmN mAN van Patented Feb. 13, 1934 UNITED STATES PATENT OFFICE TELEPHONE SYSTEM Thomas F. Crocker, Chicago, and Victor S. Tharp,

deceased, late of Elmhurst, Ill., by

Bessie M.

Tharp, administratrix, Elmhurst, lll., assignors, by mesne assignments, to Associated Electric Laboratories, Inc., Chicago, Ill., a corporation of Delaware ApplicaticnOctober 19, 1931. Serial No. 569,680

General description The invention has for its broad object the provision of new and improved circuits and apparatus for use in systems of the above type for supplying the best grade of toll and inter-oiiicc trunk service with a minimum amount of equipment. By employing the invention the initial cost of the exchange equipment is held to a low gure and the maintenance of the same is simplied with a consequent reduction in upkeep expenses.

One of the features of the invention is the trunking arrangement which permits the use of the inter-cnice trunks for both local and toll service in one direction and for local or outgoing service in the other direction.

Another feature is the improved combination local and toll selector which is adapted to provide the required grade of transmission and the necessary supervision in both classes of connections. l

Other features relate to the improved tollcord circuit of the universal type in which the discriminating circuit and supervisory circuit are both completed over the sleeve conductor.

The foregoing features together with others not specifically mentioned will be described in detail hereinafter, reference being had to the accompanying drawings.

By way of illustration it has been chosen to show the application of the invention to a multioffice telephone system consisting of two interconnected private automatic telephone exchanges. It is to be understood, however, that the use of the invention is not limited to the particular arrangement shown nor to systems of the type chosen to illustrate the same. Systems of this type are commonly known as private exchange systems and may, as in the present case, bc interconnected with an exchange of the public telephone system. When connected with a public system it is the usual practice to terminate all of the public exchange' trunks in one of the private exchanges which serves as a tandem switching center for all calls between the two systems. It is also customary to intercept all incoming calls from the public exchange system as it is not practical to list the numbers of the private exchange subscribers in the public telephone directory. The present system follows along conventional lines with regard to connection with the public exchange system.

The two offices constituting the system under consideration will be referred to hereinafter as the attended oice and the unattended ofce, respectively, and the public exchange with which connection is made will be referred to as the central oflice. The trunks from the central ofice, of course, terminate in the `attended oflice. This oflice is provided with an attendants switchboard at which point the incoming calls are intercepted and, by means of the circuits and apparatus provided, are further extended to the wanted private exchange subscriber. The attended oice is operated on what is generally known as a full multiple basis, that is, the irl-.- coming calls from the central office are intercepted by the attendant and extended to the subscribers directly through multiple jacks at the attendant's switchboard. Calls from the central oce for subscribers of the unattended oice are intercepted by the attendant and are completed over an inter-oice trunk and a switch train which in the present case serves for both the toll and local connections. It may be noted at this point that all calls from the central oflice will be referred to as toll calls, Whereas calls between subscribers in dierent ones of the private offices will be referred to as trunk calls, and calls between subscribers located in the same oflice andfnot involving the use of an inter-cnice trunk will be referred to as local calls.

Description of drawings Referring now to the accompanying drawings comprising Figs. 1 to 9, inclusive. Figs. 1 to 5, inclusive, show equipment located in the attended oice, and Figs. 6, 7, and l8, show equipment located in one of the unattended ofces. Fig. 9 is a layout diagram showing the manner in which the sheets of drawings should be arranged in order to show the inter-relation of the various circuits.

In Fig. 1 is shown the trunk relay group TR terminating a trunk line TL incoming from the central office to the attended office. 'Ihe answering jack AJ, night jack NJ, night key NK, line lamp L, and busy lamp BL appear on the attendants switchboard at which all incoming calls over the trunk line are answered. Outgoing calls are made over the branch appearing in the lower right-hand part of the sheet which extends t'o the banks of the selectors in the olce.

Fig. 2 shows a cord circuit CC for the attendants switchboard. This cord circuit is arranged for connecting directly with the multiple jacks of subscribers lines and also with trunk lines extending to the unattended ofl'ices of the system.

Fig. 3 shows the position equipment PC for the switchboard. This equipment is common to a number of cord circuits including the cord circuit CC of Fig. 2. f

In Fig'. 4 is shown a subscribers line and substation SS and the line multiple jack SJ which appears on the attendants switchboard. The relay group TR1 is associated with the trunk line TLl which extends from the attended ofce to one of the unattended oices of the system. Jack J appears on the attendants switchboard and provides means of access to the trunk for toll connections to the unattended office. f1. branch of the trunk line extends through repeater R, shown just below the trunk relay group to the switching apparatus of the attended of- Fig. 5 shows the arrangement of the switching apparatus of the attended ofce. This apparatus has been shown only in skeleton form as it consists of standard and well known units of equipment. IC is an incoming selector of the usual Strowger type terminating the branch of the trunk line from repeater R (Fig. 4). LS is a local selector and may be similar to selector IC. These two selectors have access to the trunk line extending to the central office and to local connectors, such as LC. Selector LS in addition has access to the trunk line extending to the unattended oiice. Connector LS is an ordinary Strowger type connector and has aecess to subscribers lines including the lines SS and SSl. PLS is a primary lineswitch of any suitable type terminating the line from substation SS (Fig. 4).

In Fig. 6 is shown an incoming toll and local selector TLS which is located at the unattended office. The trunk lines from the attended ofce terminate in switches of this type and are accessible to selectors in the oflice.

In Fig. 7 is shown a krepeater R1 which is associated with the trunk line TLl terminating in the selector TLS (Fig. 6). SS2 is an ordinary automatic substation telephone set associated with the subscribers line terminating in primary lineswitch PLSl. The primary lineswitch which may be of any suitable type has access to local first selectors of which one, selector LS1 is shown in skeleton form. The selector which is of the usual Strowger type has access to the inter-cnice trunk lines by Way of repeaters, such as repeater R1, and also to local connectors LC1.

Fig. 8 shows a combination toll and local connector TLC which is accessible to the incoming selectors, such as selector ILS (Fig. 6) and which has access to subscribers lines of which the one terminating in substation SSB has been shown. LC1 is a local connector similar to connector LC (Fig. 5) and is accessible to the rst selectors of the exchange, such as the selector LS1 (Fig. 7). Connector LC1 has access to the various subscribers lines of the system in com mon with connector TLC. y

Detailed description The invention having been described generally, the operations involved in establishing various classes of connections in the system shown will now be described in detail to better illustrate the various features of the invention.

Call from central oice to unattended ojice Consideration will rst be given to a call from the central oflice to station SSS whose line terminates in the unattended office. It will be assumed that the trunk line TL is the trunk line on which the call arrives. High resistance relay 106 which is normally bridged across the trunk conductors by way of contacts 120, 126, and 146 operates from battery on the upper trunk conductor and ground on the lower trunk conductor applied thereto at the central oflice upon the seizure of the trunk. Relay 106 at contacts 123 completes a circuit for relay 111. Relay 111 operates, prepares a holding circuit for relay 110 at contacts 141 and at contacts/139 connects ground to release trunk conductor 154 to mark the trunk line busy in the banks of the selectors. Busy lamp BL is connected in multiple with conductor 154 by Way of contacts 149 and the lamp glows to indicate the busy condition of the trunk line. When the operator at the central office desires to have the call further extended, ringing current is applied to the trunk line and relay 108, which with condenser 113 is connected in parallel with relay 106, operates from the ringing current. Relay 108 closes an additional point in the circuit of relay 111 to maintain that relay operated in case relay 106 restores during the ringing period. Relay 108 also completes a circuit for relay 110 at contacts 128. Relay 110 now operates and completes a locking circuit for l' itself at contacts 137 and at contacts 138 completes a circuit for line lamp L. The line lamp glows to notify the attendant that a call on the trunk line is awaiting attention.

on noting the lighted condition of 1amp L the attendant will connect her position equipment to an idle cord circuit, for example the cord circuit CC, by operating the talking key K to the left. The answering plug P of the cord circuit will then be inserted into the answer jack AJ. When the plug is inserted into the jack, contactsA 151 close and relay 112 operates. The holding circuit for relay is opened at contacts 143 and a circuit for holding relay 109 is prepared at contacts 144. relay 111 is completed at contacts 145 and the bridge including relays 106 and 108 is disconnected from the trunk by the opening of contacts 146. A circuit may now be traced from ground at contacts 157 of a trunk relay group, tip spring of the jack AJ, tip of the plug P, contacts 215, upper left-hand winding of repeating coil RC, upper winding of relay 203, contacts 220 and 223, lower winding of relay 203, lower lefthand winding of repeating coil RC, contacts 216, ring contacts of the plug and jack, contacts 136, winding of relay 104 to battery. Relay 104 operates over the foregoing circuit, connects ground to the release trunk conductor at contacts 117, and bridges the battery feed impedance 103 across the trunk conductors at contacts 118 and 119. As previously stated, upon the seizure of the trunk line at the central ofce, ground is connected to the upper trunk conductor and this ground by way of relay 105, contacts 118 and the upper winding of impedance 103 now energizes relay 105. Relay 105 completes a circuit for relay 107 at contacts 122. Relay 107 operates, closes an additional point in the circuit of relay 111 at contacts 125, and at contacts 124 A new holding circuit for l completes a circuit for relay 109. Relay 109 operates, connects the ring and tip springs of the jack through to the trunk conductors at contacts 130 and 135, completes a holding circuit for itself at contacts 134, opens the circuit of relay 104 at contacts 136, and completes a short circuit around relay 105 at contacts 131. Following the operation of relay 104, condenser 115 is bridged across the talking conductors of the trunk by Way of contacts 121 and 147. This prevents objectionable noises in the operators receiver when the cord circuit is connected through to the trunk by the operation of relay 109. Relay 104 restores after a short interval, disconnects impedance 103 from across the trunk conductors, and opens a point in the bridge above referred to. Relay 105 restores and opens the circuit of relay 107 which also restores. Relay 107 opens the initial operating circuit of relay 109 which, however, remains locked up by way of contacts 134 and 144. Relays 104, 106, and 110 are restored to normal following the operations above outlined and the attendant is connected in talking relation with the calling operator or party at the central office. Referring now to the cord circuit. Upon the operation of the key K to the left, it being assumed that the operators position circuit is not connected to any other cord circuit at this time, relay 208 will orate over its lower winding, contacts 249, conductor 264contacts 323 to ground. Relay 208 on operating completes the holding circuit for its upper winding over contacts 242 and 248, conductor 263, winding of relay 304 to ground. Relay 304 on the position equipment operates and opens the original energizing circuit of relay 208 at contacts 323 so as to prevent the operation of any other relay, such as 208, in case the talking key of another cord circuit is operated at this time. Relay 208 also loops the talking strands of the cord through the position circuit by closing contacts 239, 240, 243, and 245 and opening contacts 241 and 244. A closed circuit for the calling end of the cord may now be traced from the tip of plug P1, contacts 256 and 240, conductor 261, contacts 330, conductor 262, contacts 239, upper right-hand winding of repeating coil RC, condenser 210, and the parallel circuit including the two windings of relay 205, and contacts 231 and 234, lower right-hand winding of the repeating coil, contacts 245, conductor 259, contacts 333, conductor 260, contacts 243 and 257 to the ring contact of the plug P-1. The operators telephone set is bridged across this circuit by way of contacts 328, 322, and 318, right-hand winding `of coil IC, and contacts 313 and 321. The operators transmitter is connected in a local circuit including the left-hand winding of the coil, and the receiver is bridged across the lower half of the right-hand winding of the coil in accordance with the usual arrangement for eliminating side tone. The attendant may now converse with the calling subscriber via repeating coil RC which connects the calling and answering ends of the cord and determine the particular line in the private exchange system with which connection is desired.

As the present call is assumed to be for the substation SS3, the attendant will be so advised and will accordingly insert the calling plug P1 of the cord circuit into the jack of an idle trunk line, for example the trunk line TL1. As the calling plug P1 of the cord circuit enters the jack J, jack spring contacts 440 close and ground applied by way of these contacts, contacts 4 16 and conductor 434 energizes cut-011 relay 441 of repeater R. Relay 441 disconnects the conductors Aof the trunk line TL from the line relay of the incoming selector IC (Fig. 5) by opening contacts 450 and 453. 'Ihe closure of contacts 451 and 452 is without alect. With the plug fully inserted in the jack the loop circuit of the calling end of the cord is extended over the tip contact of the jack, contacts 407, and lower winding of relay 401 to battery and also over the ring contact of the jack, contacts 411, and resistance 430 to ground. Relay 401 operates, completes a holding circuit for its upper winding at contacts 409, completes the cir7 cuit for relay 404 at contacts 410, disconnects its lower Winding from the tip and ring springs of the jack at contacts 407 and 411, and at contacts 408 and 412 connects the springs to the talking conductors of the trunk. The cord circuit loop is accordingly extended to the line relay of the combination selector TLS. Relay 404 operates upon the closure of its circuit, connects ground to the upper trunk conductor at contacts 417, and at contacts 418 completes a circuit for slow-to-operate relay 405. Differential relay 605 and impulse relay 606 of selector TLS operate at this time from ground at contacts 417, trunk conductor 428, contacts 623, upper left-hand winding of repeating coil RC1, contacts 629 and 635, upper windings of relays 605 and 606, and contacts 659 to battery. Returning now to the trunk relay group, relay 405 operates a time after its short circuit is closed by relay 404, connects ground to cut-01T conductor 434 at contacts 419, closes a circuit for the upper winding of polarized relay 402 at contacts 420, connects ground to release trunk conductor 435 at contacts 421 to mark the trunk line busy in the banks of the local selectors, completes one holding circuit for itself at contacts 422, and completes a second holding circuit for itself at contacts 423. The circuit of relay 404 is opened at contacts 424 and this relay restores after a short interval and removes the ground from the upper trunk conductor by opening contacts 417. The original energizing circuit of relay 405 is opened at contacts 418, but the relay is now held operated over the two locking circuits previously referred to. The line relay of selector TLS 1s now held operated over the cord circuit loop which includes ther previously traced circuit including the upper talking conductor of the trunk, cord circuit loop, ring spring of the jack J, contacts 412, lower winding of polarized relay 402, conductor 429, contacts 626, lower left-hand winding of repeating coil RC1, contacts 631, 636, and 648, lower windings of relays 605 and 606, normally closed contacts 693 of the eleventh position cam springs, to ground. Differential relay 605 restores due to the energization of its lower winding and opens the initial circuit of relay 613 which, however, is held over its locking circuit. Relay 402 of the trunk relay circuit is not operated at this time as the current flow in its lower winding is such that the two windings of the relay oppose each other. In the cord circuit relay 205 is energized in series with the line relay of the selector over the previously traced circuit and opens one point in the short circuit around relay 207 at contacts 229. The opening of contacts 230 at this time is without effect as marginal relay 206 is not operated in trunk connections.

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If for any reason the attendant wishes to release the connection without extending it any further, the plug is Withdrawnfrom jack J, whereupon contacts 440 are opened and relay 401 restores. Relay 401 by closing contacts 413 connects relay 406 across the trunk conductors, since relay 405 is also operated and contacts 425 are closed. Relay 406 operates and at contacts 427 opens one of the holding circuits of relay 405. The second holding circuit for relay 405 is opened at contacts 410 upon the release of relay 401 and relay 405 restores. Relay 405 on releasing restores the trunk relay equipment to normal condition and at contacts 425 disconnects relay 406 from across the trunk conductors.

Referring now to selector TLS (Fig. 6), it will be recalled that differential relay 605 and impulse relay 606 operated upon the extension of a connection to the trunk by the operator. Relay 606 completes a circuit for relay 609 at contacts 641. Relay 609 grounds the local holding circuit by closing contacts 654, disconnects cut-oi relay 601 from the release trunk of the branch of the trunk extending to the selector banks at contacts 655, and at contacts 656 connects ground to that conductor to marl; the

, trunk line busy. The impulsing circuit is prepared at contacts 657 and the release magnet circuit is opened at contacts 658. DiiTerential relay 605 which operated during the interval that ground was applied to the upper trunk conductor, closes a circuit at contact 639 for relay 613. Relay 613 operates and locks to ,ground on the local holding circuit at contacts 679. Relay 613 remains operated over the above V circuit throughout the call and adapts the connector for toll operation by completing various circuits which will be described in detail hereinafter. Relay 602 is bridged across the trunk conductors in series with condenser 619 by the closure of contacts 675; an operating circuit for relay 603 is prepared at contacts 676 to permit that relay to operate upon the seizure of the called line by the connector; a circuit is prepared for supervisory relay 604 at contacts 678; and resistances 621 and 622 through which the upper and lower windings of the battery feed relay 614 are normally connected to the wipers of the switch, are short circuited by the closure of contacts 680 and 681. Relay 605 restores to normal upon the removal of ground from the upper trunk conductor and the completion of the loop circuit previously described for maintaining relay 606 operated.

The selector is now prepared to receive the first series of dialled impulses. The operator, accordingly, proceeds to extend the call by dialling the number of the wanted subscriber. Referring now to the cord and position circuits, when the calling device CD is turned olf-normal preparatory to transmitting the iirst series of impulses, off-normal contacts 335 close and relay 305 operates. Relay 305 at contacts 326 completes a circuit for relay 303 and at contacts 327 completes the circuit for relay 306. These relays operate and complete various circuits'as follows: Condenser 329 is bridged across the right-hand windings of the repeating coil RC of the cord circuit by the closure of contacts 325. This is for the purpose of absorbing any current discharge when the previously described cord loop is opened at contacts 330 and 333. The ring and tip contacts of the plug which are normally connected to the repeating coil of the cord circuit by way of the position equipment are now connected in a loop circuit which may be traced from conductor 260, contacts 334, and 332, impulse contacts 337 of the calling device and contacts 331 to conductor 261. The line relay of selector TLS is consequently held operated over the loop circuit including the mpulse contacts of the calling device. The operators telephone set is disconnected from the talking conductors of the cord circuit by the opening of contacts 321 and 322. The opening of contacts 328 which occurs a short time before the opening of the contacts 321 and 322 insures against the operator receiving undesirable noise in her receiver during the switching over of the connection to dialling condition. As the calling device returns to normal impulse contacts 337 are opened in accordance with the digit for which the dial was set and the loop circuit for relay 606 is interrupted accordingly. Relay 606 responds to the interruptions in its circuit by deenergizlng and reenergizing. Each time the relay restores a circuit is closed by way of contacts 642 and 657, relay 610 to battery, and in parallel therewith, through contacts 669 and vertical magnet 616 to battery. Relay 610 which is slow-to-release, operates in response to the first impulse received andA remains operated throughout the series of impulses. The vertical magnet energizes responsive to each impulse and vraises the Wipers of the switch step-by-step to the level indicated by the digit called. Relay 610 short circuits the upper left-hand winding of repeating coil RC1 at contacts 661 to rn'prove the impulse transmission, .prepares a circuit for relay 612 at contacts 663, and at contacts 662 opens a point in the circuit of the rotary magnet. Upon the rst vertical step of the wipers oir-normal contacts 696 close and a circuit may now be traced from battery, contacts 659, contacts 696', relay 612, contacts 663, 668, and 656 to ground. Relay 612 operates, prepares a. circuit for the rotary magnet at contacts 673, and completes a locking circuit for itself by way of contacts 674 and interrupter contacts 659 of the rotary magnet. Relay 610 also completes a circuit for relay 607 at contacts 664, and the latter relay operates and locks to ground on the local holding circuit by way of contacts 643. Relay 607 remains locked up throughout the connection and short circuits the two windings of the differential relay 605 by closing contacts 644 and 645.`

This is for the purpose of improving the impulsing and transmission characteristics of the switch. Shortly after the last impulse of the series relay 610 will restore and a circuit may then be traced from ground at contacts 656, contacts 668, 662, and 673, winding of rotary magnet 617 to battery. The rotary magnet operates and steps the switch wipers in on the first set of contacts in the called level and, at its interrupter contacts 659, opens the locking circuit for relay 612. Relay 612, accordingly, releases and opens the rotary magnet circuit at contacts 673. Further operations will depend on whether the trunk terminating in the first set of contacts is idle or busy. Assumingv that the trunk is busy, test wiper 690 Will find ground on the test contact and relay 612 will re-operate from this ground by way of the test wiper, contacts 666 and 659, relay 612, off-normal contacts 696', and contacts 659 to battery. Relay 612 upon re-operating again closes the circuit for the rotary magnet which operates to advance the switch Wipers another step. This stepping action is repeated until an idle trunk is found or until the switch wipers are stepped to eleventh position. In the latter event, eleventh position cam spring contacts 693 and 624' will be opened and contacts 694 will be closed. The ground connection to the lower winding of relatl 606 is replaced by a connection of ground on which busy tone is superimposed and this tone' will inform the calling party that no idle trunks are available at this time. The opening of contacts 624' prevents the operation of the switching relay 611 so that the operator will not be given line seizure supervision under these conditions.

It will be assumed that connector TLC (Fig. 8) is the rst idle connector in the group in which selector TLS is hunting. The test wiper of the selector nds no ground on the test contact and relay 612 therefor remains unoperated and relay 611 operates over a circuit traced from ground, contacts 656, winding of the relay, contacts 624 and 659, winding of relay. 612, contacts 696 and 659 to battery. Relay 611 is a high resistance relay and operates in the foregoing circuit, but relay 612 does not operate. Relay 611 connects the talking conductors of the switch through to the upper and lower wipers by closing contacts 655 and 672, connects the test wiper to its lower terminal by closing contacts 667, opens the circuit of the vertical magnet at contacts 669, completes a circuit for the lower winding of differential relay 615 at contacts 670, and completes a circuit over wiper 691 to ground by Way of contacts 671, 639', and 652. The line relay of the connector will now be energized over a circuit which may be traced from battery, upper winding of the relay, contacts 815, wiper 689, contacts 665, 640, and 646, upper right-hand winding of repeating coil RC1, contacts 684, upper winding of relay 615, lower right-hand winding of repeating coil RC1, contacts 653 and 672, wiper 692, contacts 817, lower winding of relay 802 to ground. Relay 615 of the selector does not operate at this time as the current ow in the upper winding of the relay is in a direction which causes the two windings of the relay to oppose each other. In the connector, relay 802 operates and a circuit is closed for relay 804 at contacts 819 and the relay operates to prepare the connector for operation. Ground is applied to the local holding circuit at contacts 826 and the impulsing circuit is prepared at contacts 828. Relay 805 now operates from ground on the local holding circuit, off-normal contacts 831, winding of the relay to battery. Relay 805 prepares a locking circuit for itself by closing contacts 833, completes a circuit for ring cut-off relay 806 by closing contacts 834, and connects vertical magnet 810 to the impulsing circuit by way of contacts 836.

The attendant now dials the next digit of the called number and relay 606 responds to the dialled impulses as in the previous instance. Impulses are transmitted by way of contacts 642 to operate relay 610 and this relay operates to short circuit the upper left-hand winding of the repeating coil by closing contacts 661. The previously traced circuit for relay 802 of the connector is interrupted at contacts 640 and the circuit for the lower Winding of relay 615 is likewise interrupted at contacts 641. Relay 615, therefore, remains in normal position. In the connector, relay 802 responds to the impulses and at each retraction of its armatures completes a circuit from ground, contacts 818, 820, 828, and 849, winding of relay 808 to battery, and in parallel therewith, by Way of contacts 864 and 836 to vertical magnet 810 and battery. The vertical magnet operates in response to each impulse and raises the wipers step-by-step to the level designated by the digit dialled. Relay 808 operates in response to the rst impulse received and due to its slow-releasing characteristics remains energized throughout the series of impulses. Relay 808 has only one function to perform at this time, namely, the completion of the holding circuit for relay 805 by way of con-- tacts 833 and 854. Shortly after the last impulse of the series, relay 808 restores and opens the holding circuit for relay 805. The original energizng circuit of relay 805 is opened at off-normal contacts 831 upon the flrst vertical step of the wipers and upon the release of relay 808, relay 805 restores. Relay 805 opens the energizing circuit of relay 806 at contacts 834 and relay 806 is now held operated over a circuit including its own contacts 844, contacts 835, the ring control conductor, wiper 691 oi the selector, thence to ground at contacts 652 over a circuit previously described. Relay 805 also connects up rotary magnet 811 in place of vertical magnet 810 by opening contacts 836 and closing contacts 837.

The attendant now dials the third and last digit of the called number and relays 606 and 802 respond as for the previous digit. The impulses, however, are now transmitted to rotary magnet 811 which rotates the wipers of the connector step-by-stepto the contacts of the called line. Relay 808 again operates and during the receipt of the impulses short circuits contacts 849 of busy relay 807 at contacts 855 to prevent interference with the impulsing by momentary operations of the busy relay as the wipers of the connector pass over busy contacts.

The further operation will depend on the idle or busy` condition of the called line. Assuming that the called line is busy at this time, test wiper 858 will nd ground on the test contact of the line and this ground by way of contacts 853 and 860 `,energizes busy test relay 807. Relay 807 prepares a locking circuit for itself at contacts 848 which circuit is completed at contacts 851 upon the release of relay 808 a short time after receipt of thelast impulse of the series.

Relay 807 opens the impulsing circuit of contacts 849 to prevent any further movement of the switch wipers and at contacts 850 connects busy tone to the upper trunk conductor. This tone informs the attendant that the wanted line is busy.

If the called line is idle when the connection is extended thereto as above described, there will be no ground on the test contact and consequently relay 807 will not operate. When relay 808 restores shortly after the last impulse of the series, relay 809 will operate from battery through the cut-oir relay of the lineswitch associated with the called line, test wiper 868. contacts 852, lower winding of relay 809, contacts 847 and 825 to ground on the release trunk conductor at the selector TLS. Relay 809 on operating connects ring-back-tone to the upper trunk conductor at contacts 856, connects the talking conductors of the switch through to Wipers 867 and 869, respectively, at contacts 857 and 866, completes a circuit for relay 801 at contacts 858, applies ground directly to the test wiper at contacts l859, to hold up the cut-oft relay of the lineswitch and to mark the line busy to othc1 connectors, opens the circuit of the busy test relay at contacts 860, completes a holding circuit for its own upper winding at contacts 862, opens a point in the impulsing circuit at contacts 864, and prepares a circuit for relay 803 at contacts 865. Relay 801 operates and locks to ground on the release trunk conductor by way of contacts 816. Ground is applied to the local holding circuit at contacts 813 to maintain relay 809 operated, line relay 803 is disconnected from the trunk conductors at contacts 814 and 817, and ground is disconnected from the armature of the line relay at contacts 818 to prevent the release of the switch at this time. Relays 802 and 804 deenergize with no effect at this time.

In the toll selector, the affect of the disconnection of the line relay of the connector from the trunk conductors is to deenergize the upper winding of differential relay 615 and the relay then operates over its lower winding. Relay 615 locks by way of its lower Winding, contacts 687, and off-normal contacts 696 which closed on the first vertical step of the wipers of the switch. A circuit is closed at contacts 683 for relay 603, battery feed relay 614 is connected across the talking conductors of the trunk at contacts 685 and 688, and the upper winding of relay 615 is disconnected from the trunk at contacts 684. Relay 603 operates and reverses the connection of the conductors of trunk line TLl with respect to relay 606. The reversal of current iow over the trunk line causes the two windings of polarized relay 402 of the trunk relay group TR1 to assist each other and the relay operates and closes contacts 414 to bridge relay 406 across the trunk conductors. Relay 406 operates, opens one of the holding circuits of the relay 405 at contacts 427 and at contacts 426 applies ground through resistance 431 to the sleeve contact of jack J. In the cord circuit no operation takes place until the talking key is restored to normal at which time the short circuit is removed from around relay 207 by the opening of contacts 250 and that relay operates in series with relay 206 from ground on the sleeve of the jack. Relay 206 is marginally adjusted and does not operate at this time due to the presence of resistance 431 in the sleeve circuit. Relay 207 on operating completes a circuit for lamp CL by way of contacts 238 and 236 and the lamp glows to indicate to the operator that the connection has been extended to the called line.

The operator may release theconncction at this stage by merely withdrawing the plug of the cord circuit from the jack whereupon relay 401 of the trunk circuit restores and opens the remaining holding circuit for relay 405. Relay 405 restores and the release of the connection is the same as previously explained.

When the attendant is ready to extend the connection to the called subscriber she initiates the ringing by operating ringing key RKl of the cord circuit, thereby applying ringing current to the tip and ring contacts of the plug. This ringing current is transmitted over the talking conductors of the trunk and operates relay 602 which, as previously explained, is bridged across the trunk conductors upon the operation of relay 613. Relay 602 closes a circuit for relay 608 at contacts 627. Relay 608 operates and at contacts 652 removes ground from ring control Wiper 691, thereby permitting ring-cut-off relay 806 of the connector to restore. Ringing current is now applied to the called line over a circuit which may be traced from the conductor designated Int Gen, upper winding of relay 806, contacts 840, and 867, wiper 857, line conductor 870, through thc bells and condenser at the called substation, conductor 872, wiper 869, contacts 866 and 846 to ground. The ringing current is applied to the line intermittently according to the usual practice and, when the called subscriber answers by removing his receiver from the switchhook, a direct current loop is closed ove1 which relay 806 operates from the direct current superimposed on the ringing current. Relay 806 again locks to the ring control conductor which has again been grounded by the restoration of relays 602 and 608 at the termination of the manual ringing. Relay 806 extends the talking conductors of the switch through to the Wipers at contacts 842 and 846 and battery feed relay 614 cf the toll selector now operates over the called subscribersloop. Relay 614 on operating completes a circuit from ground at contacts 682, contacts 678, relay 604 to battery. Relay 604 operates, disconnects the line relay of the selector from the trunk by opening contacts 635 and 636, and completes a. holding circuit for relay 606 by way of resistance 620 and contacts 637 and 648. As long as the called subscriber remains on the line this condition will be maintained at the selector, and the switch train will be held up independent of control from the switchboard. Talking current for the called subscriber is applied to the line through the battery feed relay 614 and the right-hand windings of repeating coil RC1 and the circuit previously traced to the called line. The trunk line between the A oiiice and the B oice is dry that is, it is not connected to either battery or ground. The talking circuit over the inter-oflice trunk also includes the right-hand winding of repeating coil RC of the cord circuit which inductively connects this talking circuit with the talking circuit completed over the incoming trunk from the central ofce.

Referring now to the cord circuit, it will be understood that the attendant will normally restore key K to normal upon the dialling ,of the last digit of the called number. The circuit of relay 208 is opened at contacts 248, this relayy restores and connects the talking strands of the cord through to the right-hand windings of the repeating coil. The operators position equipment is disconnected from the cord circuit by the opening of the various contacts controlled by relay 208 and the contacts of the key. Upon the seizure of the called line it will be recalled that the current ow over the inter-ofce trunk was reversed, as previously described, and lamp CL glowed to give line seizure supervision. The

attendant after ringing the called subscriber, as

previously described, need pay no more attention to the call unless the called subscriber fails to answer within a reasonable time. The answering of the call results in the disconnection of the line relay of the toll and local selector from the inter-cnice trunk line as above-described, thereby permitting relays 402 and 406 of the trunk relay group TR1 to restore. Relay 406 disconnects the ground from the sleeve circuit whereupon relay 207 of the cord circuit restores and extinguishes the supervisory lamp CL. The attendant is informed by the unlighted condition of the lamp that the called subscriber h answered.

The attendant may check the call at any time without actually entering into the connection by operating key K to the right or monitoring position. A circuit is closed by way of ground, contacts 252, conductor 266, contacts 324, winding of relay 302 of the position circuit, to battery, operating the relay. Relay 302 opens the operators talking circuit at contacts 313 and 317 and by closing contacts 314 and 319 bridges the left-hand winding of coil IC1 across the talking conductors of the cord. Relay 302 also bridges the receiver across the right-hand winding of the induction coil by closing contacts 316. It will be noted that the transmitter circuit is not connected in any way with the cord circuit, so that the operator cannot talk with either of the subscribers nor can the operator be overheard by the subscribers. On determining that the connection is progressing satisfactorily, the operator will normally restore the key K to normal and proceed with the establishment of other connections.

Upon the called subscriber hanging up, the circuit of the battery feed relay 614 is opened and that relay will restore and open the circuit of relay 604. Relay 604 operates and reconnects line relay 606 across the trunk conductors and polarized relay 402 of the trunk relay TR1 operates. Relay 402 again connects relay 406 across the trunk conductors, and relay 406 operates to apply ground to the supervisory circuit. Relay 207 of the cord circuit reoperates to complete the circuitof the supervisory lamp CL and the lamp glows to indicate to the attendant that the called subscriber has hung up. When the calling subscriber at the central oice hangs up, the central oflice operator will ring over thc trunk lines to operate relay 204 of the cord circuit which is bridged across the talking strands of the answering end of the cord in series with condenser 212. Relay 204 completes a circuit for supervisory lamp AL at contacts 228 and the lamp glows to give the attendant disconnect supervision. It is obvious that this lamp may also be used for recalling the attendant when desired. In this case the central ofce operator will ring over the trunk at short intervals thereby causing the lamp to flash. The supervisory lamp CL may also be used for recall by the called subscriber as this lamp can be intermittently operated or flashed by the called subscriber intermittently depressing his switch book.

After receiving the disconnect signal the attendant will release the connection by withdrawing the plugs of the cord'circuit from their respective jacks. In case the operator attempts to release a connection before the called subscriber does hang up, the withdrawal of the calling plug from jack J will be ineffective as the selector f is held under control of the called subscriber,

as previously explained. Furthermore, since under these conditions there are no battery or ground connections to the trunk line, relays 402 and 406 will be normal and relay 405 will be held operated over the circuit including contacts 422 and 427. Relay 401 will restore on the removal of the plug from the jack and will open one of the holding circuits for relay 405, but this relay will remain operated and maintain the i busy condition on the branch of the trunk accessible to the selectors. Relay 403 will also remain operated and will maintain the circuit for busy lamp L1. The continuance of this lamp to glow after the disconnection of the cord circuit will inform the operator that the called suo- 'scriber is still connected with the trunk and the operator may re-insert the plug in the jack and converse with the subscriber.

After the called subscriber hangs up and battery is reconnected to the trunk, as previously explained, relays 402 and 406 are reoperated. To release the connection under these conditions the operator withdraws the plug from the jack, thereby opening the only remaining holding circuit for relay 405 at contacts 410. Relay 405, accordingly, restores and disconnects relay 406 from across the trunk at contacts 425, opens the circuit of relay 403 at contacts 421, and opens the circuit of cut-off relay 441 of repeater R at contacts 419. The busy condition is removed from the trunk in the banks of selectors and the trunk apparatus is thereupon restored to normal condition. In toll selector TLS, the release of battery feed relay 614 upon the disconnection of the called subscriber again places the control of relay 606 with the attendant and upon the disconnection of the cord circuit from the jack and the vsubsequent; disconnect-ion of relay 406 from the trunk conductors, relay 606 will restore and open the circuit of relay 609 at contacts 641. Relay 609 releases, completes a circuit for release magnet 618 at contacts 658, removes ground from the local holding circuit at contacts 656, and opens the circuit for relays 607 and 613 at contacts 654. Relays 607, 611, and 613, accordingly, release. Release magnet 618 operates and restores the switch wipers to normal position in the usual way. During the release of the switch battery is disconnected from the upper winding of relay 606 and from relay 612 by the opening of release magnet contacts 659 in order to prevent false operation of either of these relays during the releasing operation. When the switch wipers reach normal position the release magnet circuit is opened at oil-normal contacts 695 and the holding circuit for the lower winding of relay 615 is opened at off-normal contacts 696. The release magnet and the relayv accordingly decnergize, the relay opening the circuit of relay 603 at contacts 683. Relay 603, accordingly, releases and the selector is now in normal condition and ready to receive a new call.

The removal of ground from the release trunk upon the release of relay 609 of selector TLS permits relay 801 of the connector TLC to restore to normal. Since relays 802 and 804 were released upon the seizure of the called line, a circuit will now be completed by way of contacts 818, 820, 829, and 830 for release magnet 812. Magnet 812 will, accordingly energize and restore the switch wipers to normal position in the usual way. When the wipers reach normal position the release magnet circuit is opened at offnormal contacts 830. Relay 801 on restoring also opens the holding circuit of relay 809 at contacts 813 and relay 809 restores to normal. The release of the selector, of course, opens the ring control circuit extending over wiper 691 thereupon permitting ring-cut-off relay 806 to restore to normal. The connector is now in normal condition to respond to the next call.

Call from central occ to attended o17ce Calls incoming from the central oilice for subscribers whose lines terminate in the attended oilice are extended manually by way of a multiple jack of the called line. A call of this type arriving on the trunk line TL terminating in the relay group TR (Fig. 1) is answered by means of a cord circuit, such as CC, in exactly the same manner as described for the preceding call. After determining the particular station wanted, for example station SS, the attendant will test the line for its busy or idle condition by touching the tip of the calling plug of the cord to the sleeve of the jack in which the line terminates. If the line is busy there will be ground on the sleeve of the jack and when the tip of the plug touches the jack sleeve, current will low from battery through themiddle winding of the coil IC of the operators position equipment, resistance 311, conductor 265, contacts 246, and 231, upper winding of relay 205, upper right-hand winding of repeating coil RC, upper .talking strand of the cord circuit, contacts 239, conductor 262, contacts 330, conductor 261, contacts 240 and 256, tip of the plug and sleeve of the jack to ground. The momentary current ow in this circuit produces a click in the operators receiver which informs her that the called line is busy in another connection and the operator will so inform the calling subscriber or operator.

If the called line is idle when a connection is extended, the attendant completes the connectionl by inserting the calling plug Pl of the cord circuit into the multiple jack SJ. As the plug enters the jack, spring contacts 473 close and apply ground to the sleeve of the ,jack and to release trunk conductor 474 to operate the cutoff relay of the lineswitch and to mark the line busy in the banks of the connector switches. When the plug is fully inserted in the jack ground on the sleeve is extended by way of the sleeve contact of the plug, contacts 250, winding of relay 206 to battery. This relay is marginally adjusted and now operates as ground is connected directly to its winding. Relay 206 adapts the cord to feed battery to the called line by connecting ground and battery, respectively, to the upper and lower talking strands of the cord by way of contacts 232 and 233. The attendant then signals the called line by operating ringing key RKl which applies ringing current to the line. When the called subscriber answers, relay 205 operates over the called subscribers loop, opens the initial energizing circuit of relay 206 at contacts 229 and opens the supervisory circuit at contacts 230. The short circuit is removed from relay 207 upon the operation of relay 205 and the restoration of key K to normal and relay 207 accordingly operates in series with relay 206 although this has no utility in the present connection. The attendant may supervise or monitor the call if desired, by operating key K to the right whereupon her receiver is connected to the talking strands of the cord in the manner previously described. When the connection is to be terminated and the called subscriber hangs up, relay 205 restores and cornpletes a circuit for supervisory lamp CL at contacts 230. Lamp CL glows to give the attendant disconnect supervision and `the connection is released by withdrawing the plugs of the cord from their respective jacks. 1

lnter-ojice trunk call from the attended ojce The operation of the equipment in calls bletween subscribers located in dierent offices of the private exchange system will now be described. It will be assumed for this purpose that the subscriber at station SS is calling the subscriber at station SS3. Lineswitch PLS operates responsive to the initiation of the call at substation SS and extends the calling line to an idle rst selector, such as the selector IS, which is reached over the trunk comprising conductors 561--503` The subscriber then dials the first digit of the called subscribers number, in this case the digit which designates thc unattended oilice of the system and the wipers of the selector are raised to the level in which the trunks extending to that oflice are accessible. The selector then rotates in search of an idle trunk and whenvone is found, connects the line through to the repeater associated with the seized trunk. It will be assumed that the trunk selected in this case is the one comprising conductors 470-472 which extends to repeater R (Fig. 4). The calling subscribers loop is accordingly extended over conductors 470 and 472. normally closed contacts 464 and 466, through the upper and lower windings of relay 446 to battery and ground, respectively. Relay 446 operates, completes a circuit for relay 444 at contacts 461 and closes a point at contacts 463 in the control circuit which will be extended over the inter-office trunk line. Relay 444 operates, completes another point of the control circuit at contacts 456, applies ground to release trunk conductor 471 and to conductor 435 at contacts 457, completes a circuit for cut-off relay 441 at contacts 458, and at these same contacts completes a circuit for the lower winding of polarized relay 445. Relay 445 does not operate over its lower winding alone or when its two windings oppose each other as will be pointed out hereinafter. Relay 441 operates, disconnects the trunk conductors from the incoming selector at contacts 450 and 453 and at contacts 451 and 452 connects these conductors through to the control circuit of the repeater. In the trunk relay group, relay 403 operates and at contacts 415 completes a circuit for lamp Ll which glows to indicate the busy condition of the trunk line.

A circuit for operating the line relay of the selector TLS may now be traced from battery, contacts 659, upper winding of the relay, upper winding of relay 605, contacts 635 and 629, up-

per left-hand winding of repeating coil RC1,.

contacts 623, conductor 428, conductor 436, contacts 451,` 456, and 454, upper winding of polarized relay 445, contacts 468, lower Winding of impedance coil 442, contacts 463 and 452, conductor 433, conductor 429, contacts 626, lower left-hand winding of repeating coil RC1, contacts 631, 636, and 648, lower windings of relays 605 and 606, and contacts 693 to ground. Relay 606 of the selector operates in this circuit and prepares the switch for operation as in the previous case. Relay 605 does not operate at this time since both of its windings are included in the previously traced circuit and these windings oppose each other. Consequently in the persent connection relay 613 is not operated and the selector is adapted to operate as a local selector. Polarized relay 445 of repeater R does not operate in the abovetraced circuit due to the direction of current flow in its upper winding which causes the two windings of the relay to oppose each other.

The calling subscriber now dials the second digit of the called number, interrupting the loop circuit over which relay 446 is operated and relay 446 responds in the usual way. Each time relay 446 4restores the above-traced circuit extending to relay 606 of selector TLS is opened at contacts 463 and a circuit is closed for relay 443 at contacts 462. Relay 443 which is slow-torelease operates on receipt of the first impulse and remains operated throughout the series of impulses. The function of this relay is to short circuit the upper Winding of relay 445 and the lower winding of impedance 442 to reduce the resistance of the operating circuit and thus lmprove the iinpulsing characteristics of the circuit. In the selector, relay 606 responds in the usual way and transmits the series of impulses by way of contacts 642, 657, and 669 to operate vertical magnet 616 in accordance with the digit dialled. Relay 610 operates as in the previous connection, prepares a circuit for relay 612, and at contacts 664 completes a circuit for relay 607. Relay 607 operates and short circuits the two windings of differential relay 605 by closing contacts 644 and 645 and at contacts 643 completes a locking circuit for itself to ground at contacts 654. Off-normal contacts 696 close on the first vertical step of the wipers and relay 612 operates over the circuit prepared by relay 610. Relay 612 locks by way of interrupter contacts 659 of the rotaryI magnet and its own contacts 674. When relay 610 releases shortly after the last impulse of the series, a circuit is closed from ground, contacts 656, 668, 662, and 673, rotary magnet 617, to battery. f The magnet operates and steps the wipers in on the rst set of contacts of a called level. In case the test wiper stops on a grounded contact, relay 611 is shunted and relay 612 reoperates and again completes an operating circuit for the rotary magnet. This trunk hunting operation is exactly the same as that described for the previous connection and is repeated until an idle trunk is found. It will be assumed that the trunk selected is the one4 terminating in the connector TLC. On encountering this trunk, the rotary movement of' ing of relay 802 to ground. Relay 615 of the selector does not operate at this time as its two windings oppose each other, the lower winding of this relay having been energized from ground at contacts 641, following the operation of relay 611 and the closure of contacts 670. Inthe connector relay 802 completes an obvious circuit for relay 804 which grounds the local holding circuit, completes a circuit for relay 805, and prepares the impulsing circuitasvdescribed for the previous connection.

The calling subscriber now dials the third digit of the called number. Relays 446 and 606 respond as in the previously described connection, the latter relay interrupting the circuit of relay 802 at contacts 640. Relay 802 operates and transmits a series of impulses to the vertical magnet of the connector by way of contacts 818, 820, 828, 849, 864, and 836. It may be noted at this point that relay 806 of the connector operates responsive to the operation of relay l805 and remains locked up over its own contacts 844 after oil-normal contacts 839 are opened on the first vertical step of the wipers. Relay 808 operates in parallel with magnet 810 and maintains the holding circuit for relay 805 through the series of impulses. At the end of the series of impulses, relay 808 restores and opens the circult of relay 805 which also restores as its original. energizing circuit is opened at contacts 831 upon the first vertical step of the wipers. Relay 805 connects rotary magnet 811 to the impulsing circuit at contacts 837. Relay 806 also restores as there is no ground connection t0 wiper 691 in connections of this class.

Relays 446, 606, and 802 respond as before to the fourth and last series of impulses transmitted by the calling subscribers dial, the latter relay transmitting a series of impulses over the previously traced circuit to rotary magnet 811. Rotary magnet 811 steps the wipers over the called level to the contacts of the called line, in this case the line comprising conductors 870-872 which extends to substation S53. Relay 808 operates in parallel with the rotary magnet and prepares the busy test circuit at contacts 853. In case the called line is busy wiper 868 iinds ground on the test contact and relay 807 operates. Relay 807 prepares a locking circuit for itself at contacts 848 which circuit is completed at contacts 851 upon the release of slow relay 808. The impulsing circuit is opened at contacts 849 to prevent further movement of the wipers of the switch in case the subscriber dials an additional digit. Busy tone is connected to the upper talking conductor of the connector by way of contacts 850. This tone informs the calling subscriber that the called line is busy in another connection and the subscriber is expected to release the connection.

If the called line is idle when the foregoing connection is extended thereto, busy-test relay 807 is not operated and, when relay 808 restores shortly after the last impulse of the final series, relay 809 operates in series with the cut-off relay of the lineswitch of the called line. This operating circuit for relay 809 is exactly the same as that described for the previous call to station SS3 and reference is made to that part of the description for details of the circuit. Relay 809 on operating locks to ground at contacts 826 by way of contacts 862. The impulsing circuit is opened at contacts 864, ring-backtone is connected to the upper talking conductor of the switch at contacts 856, the two talking conductors of the switch are connected through to the wipers at contacts 857 and 866, a circuit is completed for relay 801 at contacts 858, ground is applied directly to the test wiper at contacts 859, the circuit of the busy-test relay is opened at contacts 860, a circuit is closed for relay 805 at contacts 861 and a circuit is prepared for relay 803 at contacts 865. Relay 805 operates over the circuit closed by relay 809, locks by way of contacts 833 and 863 and prepares a locking circuit for the ring-cut-off relay l at contacts 834. Relay 801 operates, locks to ground on the release trunk at contacts 816, connects an additional ground to the local holding circuit at contacts 813, to take the place of the ground connected to this circuit at contacts 826, disconnects relay 802 from across the1 trunk conductor by opening contacts 814 and 817, at contacts 815 prepares a holding circuit for relay 802, and at contacts 818 removes ground from the armature of relay 802 to prevent this relay from releasing the connection. Relays 802 and 803 will now restore without affecting the connection at this time. Ring-cut-oif relay 806 having restored after the receipt of the first digit by the connector, ringing current is now automatically applied to the line over the conductors designed Int Gen, upper winding of relay 806, contacts 840 and 857, wiper 867, line conductor 870, through the bells and condenser at the called station, conductor 872, wiper 869, contacts 866 and 846 to ground. When the called subscriber answers, relay 806 operates over the called line loop, locks by way of contacts 844 and 834, and connects the called line through to the trunk by closing contacts 842 and 845. In the selector, battery feed relay 614 operates over the called line loop to which it is connected in series with resistances 621 and 622. A circuit is closed at contacts 682 for relay 603 and that relay operates to reverse the connection of line relay 606 to the trunk line TL. A circuit is also closed by way of contacts 652, 633, and 671, Wiper 691, contacts 865 and 838, upper Winding of relay 803 to battery. Relay 803 operates and closes a circuit for relay 802 at contacts 823, and prepares a holding circuit for itself at contacts 824. Original energizing circuit of relay 801 is opened at contacts 821 and an additional ground is connected to the local circuit at contacts 822. The purpose of this operation will become apparent at a later stage of the description. Referring now to the repeater R (Fig. 4) the reversal of current flow over the trunk line energizes the upper winding of relay 445 so that the two windings now assist each other and the relay operates. A circuit is closed for relay 447 at contacts 460 and relay 447 reverses the connection of line relay 446 with respect to the trunk comprising conductors 470-472. This current reversal is for the purpose of operating a meter or other apparatus which is provided in actual practice for subscribers lines in systems of this type. The bridge across the outgoing conductors and repeaters is altered to include the two windings of impedance 442 in series by the closure of contacts 469 and the opening of contacts 468. The increased resistance of this bridge reduces the transmission loss in the circuit.

At the termination of the conversation the subscribers will hang up and release the connection. hangs up iirst, the opening of the calling line loop will permit relay 446 to release and open the circuit of relay 444. Relay 444 opens the control circuit over which relay 606 of selector TLS has been held operated. Relay 606 releases and opens the circuit for relay 609. Relay 609 releases and removes ground from the local holding circuit and release trunk at contacts 656 and closes a point in the circuit of release magnet 618 at contacts 658. Relay 611 releases following the release of relay 609 and the release magnet operates to restore the switch wipers to normal position. When the wipers reach normal Aposition the locking circuit of relay 615 is opened at off-normal contacts 696 and that relay releases and opens the circuit of relay 603. When the Wipers of the selector leave thecontacts on which they have been standing, relay 614 releases and opens the circuit of relay 603 which also releases. The disconnection of the wipers from the test and ring control contacts of the trunk line opens the circuit of relays 801 and 803 of the connector. Relay 803 is slow-to-release and remains operated a short time after its circuit is opened. Relay 801, however, releases immediately and bridges relay 802 across the talking conductors of the connector before that relay has had time to deenergize as it has been operated over the local circuit including contacts 823. Relay 802 accordingly remains operated Assuming that the calling subscriber now over the called subscrlbers loop and relay 804 operates from ground by way of contacts 818 and 819. Relay 803 is also maintained energized in parallel with relay 804. When the called subscriber nally restores his receiver, relay 802 will release and close a circuit by way of contacts 818, 820, 829, and 830 for release magnet 812. Magnet 812 will restore the switch wipers to normal position in the usual way. Upon the release of relays 803 and 804 ground will be removed from the local holding circuit and relays 805, 806, and 809 will release and the switch will then be in condition to receive the next call.

Inter-oce trunk call to the attended oce A call over the trunk line TL from a subscriber in the unattended office to a subscriber in the attended oillce will now be described. It will be assumed in this example that the subscriber at substation SS2 (Fig. 7) is calling the subscriber at substation SS (Fig. 4). Lineswitch PLSl operates in the usual way upon the initiation of the call and selects an idle trunk line, for example the trunk line comprising conductors 731-733 which terminates in local selector LS1. The subscriber then dials the first or oice selecting digit of the called number and selector LS1 responds in the usual way by stepping to the level indicated by the digit dialled and searching over this level for an idle trunk line. When an idle trunk line is found, for example the trunk line comprising conductors 707-709 the selector switches through and extends the calling line through to line relay 704 of repeater R1, in which the trunk line terminates. Relay 704 operates and completes a circuit for relay 703 at contacts 718. Relay 703 closes a bridge across conductors 697 and 699 of the outgoing trunk at contacts 714, and connects ground over contacts 715 and conductor 708 to hold the preceding selector and lineswitch operated and over conductor 698 and contacts 655 to operate cut-off relay 601 of the selector TLS. Relay 703 also prepares a circuit for relay 702 at contacts 716 and at contacts 717 completes a circuit for the lower winding of polarized relay 701. Relay 601 operates and connects trunk conductors 428 and 429 to conductors 697 and 699, thereby extending the control circuit of the repeater through to the line relay of the incoming selector IC (Fig. 5) at the attended ofce. The circuit may be traced from the upper talking conductor of the selector, conductor 437, contacts 450, conductor 436, conductor 428, contacts 624, conductor 697, contacts 714 and 712, upper winding of polarized relay 701, contacts 725, lefthand winding of impedance 706, contacts 720, conductor 699, contacts 625, conductor 429, conductor 433, contacts 453, conductor 439, the lower talking conductor of the selector. relay of the selector operates over this circuit and prepares the selector for operation in the usual Way. Polarized relay 701 of repeater R1 does not operate in series with the line relay of the selector at this time as the current iiowin its upper winding is in a direction which causes the two windings of the relay to oppose each other. The selector applies ground to release trunk conductor 438 in the usual manner and this ground by way of conductor 471 which connected to conductor 438 marks the trunk line TL busy in the banks of the local selectors. Conductor 435 is also connected to conductor 438 and the ground on the latter conductor energizes relay 403 which completes a circuit for The line .ws .val

lamp L1. The lamp glows to indicate the busy condition of the trunk line. The calling subscriber now dials the first digit of the called number interrupting the calling loop in the usual way and relay 704 responds. The control circuit to the incoming selector is interrupted at contacts 720 and the line relay of that selector responds in the usual Way. Relay 702 operates in response to the rst impulse in the series and remains operated throughout the series. This relay short circuits the upper winding of polarized relay 701 and impedance 706 by closing contacts 713. The purpose of this operation is to reduce the resistance of the control circuit and thus improve the quality of impulses transmitted over the trunk line. Selector IC responds to the impulses in the usual way and then rotates to select an idle trunk line in the called level. Assuming that the trunk line 504-506 is the trunk line selected, the control circuit is extended through to connector LC. This connector operates in response to the last two digits of the called number and connects with the called line which in the'present example is assumed to be the line comprising conductors 474--476 which extends to substation SS. The connector signals the called subscriber in the usual way and applies busying ground to the release trunk conductor to operate the cut-off relay of the associated lineswitch and to mark the multiple jack of the line busy at the attendants switchboard. When the called subscriber answers the connector reverses battery over the trunk and polarized relay 701 of repeater R1 operates. Relay 701 closes a circuit for relay 705 at contacts 711. Relay 705 operates and reversesthe connection of relay 704 to the incoming trunk conductors by opening contacts 721 and 723 and closing contacts 722 and 724. At contacts 725 the left-hand winding of impedance 706 is disconnected from across the trunk conductors and the high resistance upper winding is connected in series with the left-hand winding across the trunk conductors. At the termination of the conversation the subscribers will restore their receivers and the equipment will release in the usual way.

oice from the unattended Calls from stations of the unattended oice to the central oflice are established in the same manner as calls to subscribers in the attended offices, except that incoming selector IC is operated to a level in which the outgoing trunks are accessible. For example it may be assumed that the selector IC is operated to a level in which the trunk line comprising conductors 153-155 is accessible and that this trunk line is selected. The control circuit from repeater R1 in the unattended ofilce is extended by the selector to battery through relay 104 of the trunk relay group TR (Fig. 1) by way of conductor 153 and contacts 136, and to ground at contacts 157 by way of conductor 155. Relay 104 operates and connects ground at contacts 117 to release trunk conductor 154 to maintain selector IC operated, and bridges the battery' feed impedance 103 across the trunk conductors 101 and 102 at contacts 118 and 119. Relay 105 is connected in series with the upper winding of impedance 103 and operates in series with the line relay at the central omce. The line relay at the central oillce operatesin the usual way to notify the operator of the incoming call. Relay 105 operates and completes a circuit at contacts 122 for relay 107. Relay 107 operates, completes a circuit for relay 109 at contacts 124, and at contacts 125 completes a circuit for relay lll. Relay lll applies ground to the release trunk conductor at contacts 139 to take the place of the ground at contacts 117 which will be removed shortly. Relay 109 connects the incoming trunk conductors 153 and 155 straight through to the outgoing trunk conductors lOl and 102 at contacts 130 and 135, respectively. Relay 104 is disconnected from conductor 153 at contacts 136 and the ground is disconnected from conductor 155 at contacts 157. The connection is now held up by relay 105 which is operated in series with the line relay at the central oflice and which in turn maintains relays 107 and 111 operated. The busy lamp BL glows to indicate the busy condition of this trunk to the attendant at the switchboard.

While in the present case it is assumed that the trunk line extends to a manual office, it is obvious that it may as well extend to an automatic oil'lce and terminate in a lineswitch or selector. Under these conditions the line of relay of the incoming switch will be operated and as the control circuit from repeater Rl is cxtended straight through to the trunk this line relay may be directly controlled by interrupting the control circuit at the repeater. When the connection is switched through, as described, relay 104 releases and disconnects the battery feed impedance from the trunk and the talking battery for the connection will now be supplied from the central oilice. When the calling subscriber hangs up the selectors will release in the usual way and the control circuit will be opened at the repeater. Relay 105 will, accordingly, restore followed by the restoration of relays 107l 109, and 111. Removal of ground from the release trunk conductor will permit the incoming selector IC to release and the busy lamp will be extinguished. The selectors or other equipment in the central oiiice will be released in the usual way, or the central olce operator will receive disconnect supervision and release all connections in accordance with the usual practice.

Local calls Local calls between subscribers in different oillces are completed in the usual way over the well known equipment and a detailed description is not considered necessary. The subscriber at station SS, for example, in calling station SSl, will operate the primary lineswitch PLS to connect with a local first selector, such as the selector LS. Selector LS will be operated to a level in which the local connectors are accessible and an idle local connector, such as connector LC, will be operated to extend the connection to the called line. The called line will be signalled in the usual Way and, at the termination of the connection the equipment will be released in accordance with the usual practice.

A local call in the unattended ofllce, for example, a call from station SS2 to station SSB will be established over a primary lineswitch PLSl, a local tlrst selector LS1, and a local connector LC1. These switches are of the well known Strowger type as previously stated and their operation is familiar to those skilled in the art.

Night service In systems of the type described it frequently happens that the incoming trafllc at night or on holidays is very light and consequently the attendant's switchboard is left unattended. However, in order that the exchange systems lnay not be entirely cut-oit from incoming service, one or more of the trunk lines from the central oice are connected directly to certain subscribers lines in the attended ofce. These lines, for example, may be those serving the watchman or other person who is present during the light traffic hours in question. The trunk line TL from the central ofce for example may be connected to thc line of the station SS by means of the cord circuit CC so that all incoming calls over the trunk line will be answered at the station SS. To set up a connection for night service the attendant before leaving the switchboard will insert the answering plug P of the cord circuit CC into night jack NJ of a trunk. When the plug enters the jack contacts 148 close, thereby applying ground to release trunk conductor 154 to mark the trunk line busy to selectors in the attended oflce, and at contacts 150 a circuit is prepared for relayv 100. The talking strands of the cord are connected to the talking conductors of the trunk circuit over the tip and ring contacts of the jack in the usual manner. The attendant will then insert the calling plug P1 of the circuit into the multiple jack SJ of the subscribers line. operating the cut-off relay of the lineswitch in the usual way. The attendant if'she considers it necessary may signal the subscriber and inform him that the line is being set up for night service and that it is, therefore, not available for outgoing calls. Otherwise the cord circuit is left in normal condition with only relay 206 operated. The attendant will also operate night key NK of the trunk circuit, opening the .circuit of relay 110 at contacts 152. Upon the extension of a call over the trunk circuit set up for the night service, as above described, high resistance relay 106 operates as in previous calls and at contacts 123 closes a circuit for relay 111. Relay 111 at contacts 140 completes a circuit for relay 109. Relay 109 operates and' connects the tip and ring springs of the night jack through to the talking conductors of the trunk at contacts 130 and 135. Relay 105 is shunted around condenser 114 and now operates in series with the battery feed relay at the central oilice. Relay 105 completes a circuit for relay 107 and the latter relay opens the bridge including relay 106 which is normally connected across the trunk conductors. Relay 107 also completes a holding circuit for relay 11. When the central ofce operator applies ringing current to the trunk this current is repeated through the repeating coil-RC of the cord circuit to the subscriber's line and rings the bells at the called station. When the connection is pulled down at the central cnice, relay 105 will release and permit relays 107, 109, and 111 to release. The trunk line will then be in condition to receive the next call.

What is claimed is:

1. In a telephone system, a switch train comprising a selector repeater and a connector, a trunk line terminating in the selector repeater, means for extending calls of two classes to the trunk line, means for momentarily unbalancing the trunk line responsive to the extension of a call of lone class thereto, and means in the selector repeater controlled by said momentary unbalance for determining the particular manner in which the selector repeater and connector will subsequently operate to further extend the call.

2. In a telephone system, a switch train comprising a selector repeater and a connector, a. trunk line terminating inthe selector repeater. means for extending connections of two classes tothe trunk line, means controlled over the trunk line for operating the-selector and connector to further extend the connection to a called line, means for momentarily unbalancing the trunk line responsive to the extension of a connection of one class thereto, and means in the selector repeater for determining whether or not the called line will be automatically signalled responsive to the extension of the connection Vthereto dependent on the balanced or unbalanced condition of the trunk line.

v3; In a telephone system, a switch train comprising a selector repeater and a connector, a. trunk line terminating in the selector repeater, means for extending calls of two classes to the trunk line, means for momentarily unbalancing the trunk line responsive to the extension of a 'call of one class thereto, means controlled over the trunk line for operating the selector repeater and the connector to further extend the call, and means in the selector repeater dependent on the balanced or unbalanced condition of the trunk line for determining the particular supervisory signals returned over the trunk line by the selector repeater.

4. In a telephone system, a two conductor trunk line terminating in a selector repeater, a connector, a called line, means for extending connections of two classes to the trunk line, means controlled over the two conductors of the trunk line in series for operating the selector repeater and connector to further extend the connection to said line, and means automatically controlled over only one of said trunk conductors prior to the further extension of the connection for determining whether or not the selector repeater will return a supervisory signal over the trunk line responsive to the extension of the connection to the line.

5. In a telephone system, a trunk line, two automatic switches, a source of current connected to the trunk line at therst switch, means for extending calls of two classes to the trunk line, means controlled over the trunk line for operating the switches to further extend the calls to called lines, means responsive to the extension of a call of one class to a line for reversing the connection of said current source to the trunk line, and means responsive to the answering of a call of the other class extended to a line for reversing the connection of said current source to the trunk line.

6. In a telephone system, a trunk line, two automatic switches, a source of current normally connected to the trunk line at the rst switch, means for extending calls of two classes to the trunk line, means for operating said switches to further extend the calls to called lines, means responsive to the answering of a call of one class for reversing the connection of said current source to the trunk line, means responsive to the extension of a call of the other class to a line for reversing the connection of said current 

